In the above-identified application, there is described a process for forming a localized field oxided region of relatively narrow width by a process which involves forming over a silicon substrate a succession of layers including in turn a pad thermally grown silicon oxide layer, a CVD (chemical vapor deposited) silicon nitride layer, and a CVD silicon oxide layer thereafter using photolithographic techniques for masking pattern definition as aperture with relatively vertical side walls is formed through the outer two layers, terminating at the thermally grown oxide. Next a conformed CVD silicon nitride layer is deposited in a manner to fill the aperture rising over the edges of the aperture to extend over the surface of the CVD silicon oxide layer. This layer is then etched an isotropically to remove the thickness corresponding to the conformed silicon nitride layer to reform the aperture but leaving silicon nitride side wall spacers which effectively reduce the lateral dimensions of the aperture. The resultant was then heated in an oxidizing atmosphere to thicken the thermally grown oxide exposed in the aperture to the thickness desired for the field oxide.
This technique permits achieving field oxide regions of narrower widths then the minimum size fixture realizable by standard photolithographic techniques by the width of the side wall spacers provided after the original field oxide pattern has been defined by the photolithographic process.
The present invention relates to improvements in the basic process described.